1. Field of the Invention
The present invention relates to printed circuit boards and, more particularly, to mounting structures that control stresses between electrical components and conductive surfaces on printed circuit boards.
2. Description of Related Art
In the field of electronic circuit fabrication, it has been the general practice to employ printed circuit assemblies wherein much of the circuit wiring and the circuit components are mounted on a common base. In general, a printed circuit usually comprises a relatively rigid base on which a pattern of printed wires is formed in some predetermined configuration. The printed wiring is usually etched from a previously deposited layer of copper cladding. The printed wiring generally includes narrow conductive strips called "circuit traces" and broad conductive surfaces called "pads". The traces and pads provide point-to-point electrical connections for the separately manufactured electrical components, such as resistors, capacitors, transistors, etc. The components are usually mounted on the pads by soldering or other process to produce a conductive contact between the component's terminals and the pads.
Modern-day circuit fabricators often prefer to use printed circuit boards to other structures because of the neatness and miniaturization made possible by their use. There are additional benefits to be realized from the use of circuit boards: Low lead inductance, improved physical stability of the components and interconnecting leads, and good repeatability of the basic layout of a given circuit. The repeatability factor makes the use of circuit boards ideal for mass production techniques.
There are a number of alternative construction techniques that are employed for mounting the components on the circuit boards. One simple method is to use leaded components. In this method, the component's leads are first bent into an appropriate shape and the component is then mounted on the board by soldering the bent leads to spaced pads on the board. The lead-pad interconnection provides some mechanical stability in addition to the necessary electrical coupling of the component to the printed circuit.
Another mounting method, an important space-saving technique, involves the use of surface-mounted components. These components, often referred to as chips, are used widely in UHF and microwave circuits. The surface-mounted components are usually small blocks having conductive surfaces at either end that act as terminals. When used on printed circuit boards, the conductive surfaces are soldered directly to the pads.
Although the various mounting alternatives have served the purpose, they have not proved entirely satisfactory under all conditions of service for the reason that considerable difficulty with component mounting has been experienced. Specifically, circuit failures occur frequently due to stress-induced fractures that develop at the soldered interconnections between the components and the pads. Leaded components and adhesive bonding are the main technologies currently used for stress control of small electrical components to avoid such failures. However, leaded components do not lend themselves to dense packaging or very high production rates. They are also more expensive than surface-mounted devices. Adhesive bonding is not an attractive high-rate process, is a secondary operation adding recurring costs and is limited by the adhesive properties.
As such, cost and competition are driving the electronics industry to surface mount components so as to more densely pack the components into a smaller space. Also, low-cost circuit boards fabricated from plastic materials by injection molding are now being adopted by the electronics industry to lower costs. Those skilled in these arts recognize that one of the most critical problem confronting designers of such circuits is achieving sufficient physical stability of the component interconnections on the boards to avoid stress-induced failures due to such causes as thermal expansions, vibrational fatigue, and the like.
It is, therefore, a primary object of the present invention to control stresses induced between printed circuit boards and the components mounted thereon.